Field of the Invention
The present invention relates to a thermal displacement compensation device of a machine tool which improves working efficiency upon adjustment of the amount of thermal displacement compensation amount.
Description of the Related Art
In some cases, a function of calculating and compensating a thermal displacement amount from, for example, an operation of a machine tool or a temperature of each part cannot more accurately calculate the thermal displacement amount due to surrounding environment such as a coolant or an outside air temperature, and therefore an error (compensation error) between the thermal displacement compensation amount and an actual thermal displacement amount becomes significant, and the thermal displacement amount cannot be accurately compensated. In general, when the compensation error becomes significant due to an external factor, compensation precision is improved by adjusting the thermal displacement compensation amount by increasing or decreasing the thermal displacement compensation amount.
As a first example of a method of adjusting a thermal displacement compensation amount, Japanese Patent Application Laid-Open No. 2002-18677 discloses a method of adjusting a thermal displacement compensation amount, when a calculated thermal displacement compensation amount is significantly different from an actual thermal displacement amount due to an external factor, by newly calculating a coefficient related to heat generated by spindle rotation and axis movement in a thermal displacement calculation equation based on the thermal displacement compensation amount stored in a controller of a machine tool and a compensation error.
Upon adjustment of the thermal displacement compensation amount, sections obtained by dividing an entire length of a feed screw which forms a feed axis portion into a plurality of sections are set, and a thermal displacement amount δnI of a section I at a time n is calculated according to following equation (1).δnI=δ(n-1)I+A×vnI−BnI+DnI  (1)
A: Heat generation coefficient
vnI: Value calculated from axis moving speed at time n
BnI: Contraction due to heat dissipation of feed axis per unit time in section I
DnI: Displacement due to heat conduction from a section adjacent to section I
A feed axis portion thermal displacement amount LnX in a section X at the time n is calculated by adding the thermal displacement amount in each section from a reference position to the section X as in following equation (2).LnX=δn0+δn1+ . . . +δnI+ . . . +δnX  (2)
LnX: Feed axis portion thermal displacement amount in section X at time n
When the heat generation coefficient A increases, the thermal displacement compensation amount increases, and, when the heat generation coefficient A decreases, the thermal displacement compensation amount decreases. Focusing on this, the heat generation coefficient A is calculated again using following equation (3) based on a compensation error (adjustment value ε) set and a value (T) calculated from the thermal displacement compensation amount at a time at which the compensation error is produced upon adjustment of the thermal displacement compensation amount.A′=A×(1+ε×T)  (3)
In this case, whether to increase or decrease the coefficient A (to increase or decrease the thermal displacement compensation amount) is determined based on a sign of the adjustment value ε, and an increase amount of the heat generation coefficient A (a movement amount of a machining point) is determined based on an absolute value of the adjustment value ε.
Further, as a second example of a method of adjusting a thermal displacement compensation amount, Japanese Patent Application Laid-Open No. 11-90779 discloses a method of changing and adjusting a thermal displacement compensation amount by adding or subtracting an adjustment value to or from the calculated thermal displacement compensation amount.
According to the methods of adjusting thermal displacement compensation amounts disclosed in above Japanese Patent Application Laid-Open No. 2002-18677 and Japanese Patent Application Laid-Open No. 11-90779, two elements of “increase or decrease of a thermal displacement compensation amount (a sign of an adjustment value)” and “movement amount of a machining point (an absolute value of an adjustment value)” are set.
However, a thermal displacement compensation amount represents the amount of extension of a feed axis not the movement amount of a machining point on a machine coordinate and, therefore, the sign does not match with the machine coordinate in some cases, and a fixed end of the axis and a direction of a machine coordinate differ depending on a machine tool or an axis. Hence, a thermal displacement compensation amount is compensated by adding a position command value to the thermal displacement compensation amount in some cases and a thermal displacement compensation amount is compensated by subtracting a position command value from the thermal displacement compensation amount, and therefore compensation is hardly performed adequately. As a result, adjustment of a thermal displacement compensation amount is insufficient, and therefore “increase or decrease of a thermal displacement compensation amount” does not match a moving direction of a machining point (a position at which a work is machined by a tool) and then a wrong setting is made upon a setting of adjustment of the thermal displacement compensation amount and, as a result, the thermal displacement compensation amount is decreased when the thermal displacement compensation amount needs to be increased and the thermal displacement compensation amount is increased when the thermal displacement compensation amount need to be decreased.